It is conventionally known in a drive control apparatus for a three-phase rotary machine, which has a three-phase coil set, to reduce torque ripple by superimposing n-th harmonic on a main component of an AC current (for example, JP 3804686 corresponding to JP-A-2006-158198). The n-th harmonic (harmonic of n-th degree) in this case is typically the fifth harmonic.
Torque of a rotary machine is determined by a product of a current and a magnetic flux, and hence no torque ripple arises if the product of the current and the magnetic flux does not change with phases. In a case that magnetic flux contains distortion component, however, torque ripple is likely to arise because of influence of torque corresponding to the distortion component.
In the drive control apparatus disclosed in the above-described patent document, a motor is driven by one drive system of a power converter. For example, when a fifth harmonic current having a certain amplitude is superimposed on a main component of an AC current based on the above-described patent document in a case that the magnetic flux contains the fifth harmonic distortion component, the torque corresponding to the distortion component of the magnetic flux and the torque corresponding to the fifth harmonic current act to cancel out each other.
However, depending on a relation between the amplitude (magnitude) of the distortion component of the magnetic flux and the amplitude of the superimposed harmonic current, the torque is not always cancelled out and torque ripple remains in some cases. If a peak value of a phase current is high, the power converter and the coil set tend to generate large heat.